This invention relates to a receiver for a spectrum spread communication system, and more particularly to a receiver for a spectrum spread communication system employing a direct spread modulation system.
In radio communication, it is often encountered that a radio wave generated from a transmit side arrives at an antenna on a receive side while taking the form of a plurality of incoming waves which have passed through a propagation path. Such a phenomenon is called multipath, which will be described hereinafter with reference to FIG. 5. In FIG. 5, reference numeral 101 designates a transmit antenna and 102 is a receive antenna. A radio wave transmitted from the transmit antenna 101 is received by the receive antenna 102, during which the receive antenna 102 receives both a direct wave sent thereto directly from the transmit antenna 101 through a propagation path 103 and an indirect wave sent thereto from the transmit antenna 101 through a propagation path 10 5 including reflection on a reflector 104. Actually , a wall surface of a building, a wall of a room and the like each act as the reflector 104. A plurality of indirect waves are propagated through such a propagation path 105. Such propagation of waves through a plurality of paths different in propagation time is generally referred to as "multipath" in the art.
Generation of multipath causes a variety of troubles. More particularly, in communication equipment for transmitting digital data such as radio data communication equipment, it causes a receive signal to be indefinite, leading to a failure in receive of the signal. Alternatively, it, even when the communication equipment receives the signal, causes a bit error rate (BER) to be deteriorated, leading to a deterioration in communication quality. In particular, fading due to multipath wherein a plurality of signals by multipath cancel each other to cause at least a part of a spectrum of the receive signal to be decreased in intensity gives rise to a serious problem.
Conventional approaches to such troubles due to multipath include (1) employment of a space diversity system by an antenna, (2) use of a waveform equalizer, (3) employment of a spectrum spread communication system, and the like.
Now, a space diversity system will be described with reference to FIG. 6. In FIG. 6, reference numerals 201, 202 and 203 each designate each of a plurality of antennas arranged in a manner to be spatially separated from each other. In the example shown in FIG. 6, three such antennas are arranged. 204 is an antenna switching unit and 205 is a receiver. In the space diversity system thus constructed, of the receive antennas 201 to 203 which are spatially separated from each other, the antenna increased in receive intensity is selected through the antenna switching unit 204, to thereby be connected to the receiver 205, resulting in ensuring stable receive.
Use of a waveform equalizer is carried out in such a manner as shown in FIG. 7. In FIG. 7, reference numeral 301 designates a receive antenna, 302 is a radiofrequency circuit, 303 is a waveform equalizer and 304 is a demodulation circuit. In a receiver thus constructed, a signal received by the receive antenna 301 is frequency-converted into an intermediate frequency in the radiofrequency circuit 302 and then input to the waveform equalizer 303. The waveform equalizer 303 acts as a circuit for permitting distortion of a waveform due to multipath which occurs in a signal propagation path to be corrected by means of a delay element, a variable weighting circuit or the like. It may be constituted by an A/D converter, a digital circuit or the like. The receive signal corrected in the waveform equalizer 303 is input to the demodulation circuit 304. At this time, the waveform equalizer 303 has already corrected the receive signal to a state at which it was transmitted, resulting in ensuring stable demodulation operation.
Now, a spectrum spread system will be described with reference to FIG. 8. In FIG. 8. reference numeral 401 designates a base band signal generation circuit, 402 is a direct spread modulation circuit, 403 is a spread code generator and 404 is a transmit antenna 404. The elements 401 to 404 cooperate with each other to provide a unit on a transmit side. Reference numeral 405 is a receive antenna, 406 is a direct spread demodulation circuit, 407 is a spread code generator and 408 is a base band receive circuit. The elements 405 to 408 cooperate with each other to constitute a unit on a receive side.
In the spectrum spread system thus constructed, the base band signal generation circuit 401 generates base band information to be transmitted. The base band information thus generated usually has such a spectrum as shown in (A) of FIG. 8. The spread code generator 403 generates a spread code sequence consisting of a pseudo noise (PN) code of a speed larger than the base band information and the like. The base band signal and spread code sequence thus generated are then input to the direct spread modulation circuit 402. so that the base band signal is modulated by the spread code sequence, resulting in a spread modulation signal which has a spectrum spread over such a wide band as shown in (B) of FIG. 8. The direct spread modulation signal is subject to carrier modulation such as PSK modulation. FSK modulation or the like in the direct spread modulation circuit 402 and then transmitted from the transmit antenna 404 toward the receive side.
The signal thus received by the receive antenna 405 on the receive side is input to the direct spread demodulation circuit 406. At this time, it is supposed that a spectrum of the receive signal is partially attenuated due to fading by multipath described above or the like, as shown in (C) of FIG. 8. The receive signal thus input through the receive antenna 405 while being subject to fading is demodulated in the direct spread demodulation circuit 406 and then correlated to a spread code sequence generated from the spread code generator 407 which is adapted to generate the same spread code sequence as the spread code generator 403 on the transmit side, so that a demodulation signal having such a spectrum as shown in (D) of FIG. 8 is output from the direct spread demodulation circuit 406. At this time, even when a part of the receive signal is attenuated as shown in (C) of FIG. 8, a whole spectrum of the receive signal is spread over a wide frequency band, to thereby minimize an influence of the attenuation on the receive signal. This substantially prevents the signal demodulated by the direct spread circuit 406 from being affected by fading. The signal thus demodulated is fed to the base band receive circuit 408, resulting in the base band signal being reproduced.
Unfortunately, each of the approaches described above exhibits its own disadvantages.
More particularly, in the space diversity system using the antennas shown in FIG. 6, a signal intensity detection circuit incorporated in the receiver 205 controls the antenna switching unit 204. to thereby select any desired one of the antennas. Thus, when the antenna switching unit 204 is controlled to carry out switching between the antennas at the time when the signal intensity detection circuit detects a decrease in intensity of the receive signal due to fading, reaction time required for the switching is increased because there is a signal delay due to circuits and filters between an end of the antenna and the signal intensity detection circuit, resulting in the receive signal being deteriorated between the detection and the switching. This causes a radio data communication equipment or the like to exhibit a disadvantage that data received between the detection and the switching contain an error.
The approach of FIG. 7 using the waveform equalizer is complicated in setting of a tap coefficient and calculation of a correction level. Also, it requires a logic circuit of an advanced grade, a digital filter to which a digital signal processor (DSP) is applied and the like in order to realize the setting and calculation. Further, it renders restoration of the receive signal impossible when an intensity of the spectrum is deteriorated to a level sufficient to render the correction impossible or an intensity of the signal is reduced to a level below a receive intensity limit.
Also, use of the spectrum spread system shown in FIG. 8 fails to eliminate the above-described disadvantages due to fading by multipath when a large part of the spectrum directly spread by spectrum spread or a whole thereof falls in due to fading or a length of the spread code sequence is reduced.